The 1D6 monoclonal antibody reacts with human BLyS (B lymphocyte stimulator), also known as BAFF (B cell activating factor from the TNF family), TALL-1 (TNF and apoptosis leukocyte-expressed ligand-1), THANK (TNF homologue that activates apoptosis, nuclear factor KappaB and c-Jun NH2-terminal kinase) and zTNF4. BLyS, a member of the TNF family, is a type II membrane protein that exists in both membrane-bound and soluble forms. Expression of membrane-bound BLyS on monocytes lineage is regulated by IFN gamma. BLyS acts as a potent B cell growth factor and costimulator of Ig production. BLyS transgenic mice develop severe B cell hyperplasia and autoimmune lupus-like disease characterized by the presence of autoantibodies against nuclear antigens and immune complexes deposited in the kidney. Data from animal models imply a role for BLyS in human autoimmune diseases. To date BCMA, TACI and BR3 have been identified as receptors for BLyS. APRIL, another TNF family member, and BLyS can form active heterotrimeric molecules when coexpressed and these circulating heterotrimers are present in serum samples from patients with systemic immune-based rheumatic diseases. Furthermore, it has been reported that BLyS and APRIL, share BCMA and TACI as their receptors.